The filamentous fungus Aspergillus fumigatus is one of the causes of invasive lung disease in immunocompromised individuals. It is classified as asexual because no direct observation of mating or meiosis has been reported. Sequencing of the complete genome by an international collaboration, including the Wellcome Trust Sanger Institute (UK) and The Institute for Genomic Research (TIGR, USA), has made most of the genomic sequence information from A. fumigatus publicly available. By searching the incomplete genome sequence of A. fumigatus, I have identified the coding capacity for a set of proteins that could be involved in mating and the pheromone response pathway. These include one putative mating-type gene, one gene encoding a pheromone and two pheromone-receptor genes. The mating-type gene encodes a high-mobility group domain protein exhibiting significant similarity with mating-type proteins from sexually reproducing filamentous ascomycetes. The pheromone gene is predicted to encode a precursor pheromone that is processed by a KEX2-like protease to yield a pheromone that is structurally similar to the alpha-factor of the yeast Saccharomyces cerevisiae. In addition, the deduced gene products of the receptor genes are putative seven-transmembrane proteins, which display a high-level amino acid identity with the a-factor receptor Ste3p and the alpha-receptor Ste2p of S. cerevisiae, respectively. The identification of these homologues suggests the existence of a sexual cycle in A. fumigatus.